In stagnant waterbodies such as water retained in lakes, dams, large-scale water tanks, the level of dissolved oxygen (DO) is likely to decrease as the depth of water bodies increases. The decreased dissolved oxygen arises due to consumption of dissolved oxygen that is attributable to action of aerobes or oxidation of reductive matter in water. Another reason of the decreased dissolved oxygen in deep water may be the layered structure of a waterbody. That is, a cold water mass that is located in deep water and has a high density prevents oxygen from entering into the waterbody. When the amount of organic materials and reductive materials introduced into a waterbody increases, an anoxia or hypoxia region expands throughout the waterbody, changing the food chain, endangering aquatic ecosystems, and destroying self-purification ability of the waterbody. As a result, the quality of water is deteriorated.
In order to solve this problem, several methods have been performed to increase the DO level in a middle or lower layer portion of a waterbody, thereby expanding an aerobic region in a benthic ecosystem and restoring the entire ecosystem by putting air into the lower layer portion of the waterbody such as a polluted lake.
The most frequently performed method is a method of installing an air supply pipe and an air diffuser in a lower layer of a waterbody and transferring air into the lower layer of the waterbody through the air supply pipe and the air diffuser. In this case, air bubbles form a thin air stream in water and rapidly rise to the surface of water along with water. Therefore, a retention time of air in water is very short. Furthermore, the surface contact area of air bubbles that come in contact with water is very small. In addition, mixing effect between water and air bubbles is limited. For these reasons, oxygen transfer efficiency to a waterbody is low and the method is ineffective to supply oxygen to a deep region of a waterbody in which oxygen is substantially needed. Furthermore, when a waterbody is deep, it is necessary to supply compressed air to the waterbody. Therefore, when the waterbody is deep and wide like a large lake, aeration of the waterbody is very costly.
Korean Patent No. 10-1334446 (Nov. 29, 2013), 10-1453730 (Oct. 22, 2014), U.S. Pat. No. 4,724,086 (Feb. 9, 1988), and U.S. Pat. No. 5,755,976 (May 26, 1998) disclose a technology by which a pipe duct is installed between a lower layer and an upper layer of a water system and water in either one layer is pumped to the other layer to solve the problem of a hypoxia state occurring in the lower layer of the water system. U.S. Patent Application Publication No. 2011/0147289 (published on Jun. 23, 2011) discloses a method of circulating water between a lower layer and an upper layer of a waterbody by installing a twisted rotational blade (propeller) in the waterbody instead of the pipe duct and rotating the twisted rotational blade using a driving source to form an ascending water flow and a descending water flow.
The former method, which circulates water using the pipe duct, incurs a low installation cost when it is applied to a small-scale waterbody but is very costly in terms of installation cost and operation cost when it is applied to a large-scale waterbody such as a large lake or a dam reservoir. Therefore, it is difficult to use the former method for aeration of a large-scale waterbody. The later method, which circulates water using a propeller, forms a parallel flow 13 that moves forward when a propeller 12 is rotated by a driving device 11 as illustrated in FIG. 1. At this point, since resistance that a fluid receives is increased, water conveyance efficiency is decreased when the method is used for a deep waterbody.
On the other hand, a natural river has a riffle-pool sequence because water forms a curved stream due to various natural conditions. In a pool of a curved stream, a whirlpool is formed due to the effect of the flow of water. The whirlpool erodes the bed of a river in a different amount according to the flowrate of water at the curved portions of a river during flooding. Soil, sand and pebbles that are eroded from the bed of a river accumulate in a position where the whirlpool is weak, forming a riffle there. In pools of a natural river, a whirlpool is unfailingly formed. Therefore, even when a pool is deeper, a correspondingly larger and stronger whirlpool is formed, so that surface water which is high in DO level and is rich in organic materials is circulated to reach the bottom of the pool in a river. Therefore, a good aerobic ecosystem can always be maintained at the bottom of the pool, so that even a large fish which consumes a large amount of oxygen can live deep in the waterbody.
Referring to FIG. 2, it is seen that riffles 21 and pools 22 repeatedly appear in a natural river as curved portions do. As illustrated in FIG. 3, as for the flow of water in a pool (refer to 22 of FIG. 2), a water flow 31 that flows straight into a curved portion of a river collides with the bank at the curved portion of a river, forming a whirlpool 32. The whirlpool transfers surface water in which dissolved oxygen is sufficient to the bottom of a river, thereby activating a benthic ecosystem.
When a whirlpool is artificially generated in a stagnant waterbody such as a lake, a hypoxia layer near the bottom of the waterbody is broken. Therefore, a benthic ecosystem can be restored, the total amount of phytoplankton can be controlled due to active athrocytosis of shellfish and microfauna, and nutritive components in water can accumulate and be stored in animal bodies and can be used in the food chain. Through this process, the quality of water can be improved.
Basically, an artificial whirlpool can be formed in water by forming a horizontal flow and by using a disturbing plate with which the horizontal flow obliquely collides. However, development of an apparatus that can prevent wasteful energy consumption attributable to diffusion of flow in a process of forming a whirlpool and can improve efficiency of formation of a whirlpool is still required.
As described above, in a deep waterbody such as a lake, when solving the problem of expanding anoxia and hypoxia region in the system, which increases the amount of products of anaerobic degradation and accelerates the collapse of self-purification ability of a waterbody, conventional methods of increasing the DO level in a middle or lower layer of a waterbody are likely to have the following problems.
The first method in which an air supply pipe and an air diffuser are installed in a lower layer of a waterbody and air is transferred to the lower layer of the waterbody through the air diffuser to increase the level of dissolved oxygen, has the problem of low oxygen transfer efficiency due to (a) a short retention time of air in water attributable to formation of a thin stream of air bubbles and fast ascending of air bubbles toward the surface of water along with water, (b) a limited surface contact area with water, and (c) a limited mixing and contact effect between water and air bubbles. In addition, the method is difficult to achieve the goal of supplying oxygen to a lower layer of a waterbody in which oxygen is substantially needed. Furthermore, since it is necessary to transfer compressed air when a waterbody to be aerated is deep, high cost incurs in aerating a large lake.
The second method in which a pipe duct is installed between a lower layer and an upper layer of a waterbody and water is circulated by pumping water in any one layer to the other layer using a driving source to break a hypoxia state in the lower layer is highly costly in terms of equipment installation and operation when it is applied to a large lake or a dam reservoir; however, the method only incurs a small equipment cost when it is applied to a small waterbody.
According to the third method in which a tortile rotational blade (propeller) is installed instead of a pipe duct in water and water is circulated between a lower layer and an upper layer by forming an ascending flow or a descending flow by rotating the propeller with driving force, parallel flows are formed when the propeller is operated, greatly increasing resistance that a flowing waterbody receives. Therefore, when this method is applied to a deep waterbody with a depth of several tens of meters, water circulation efficiency is too low to apply to water treatment.
In addition, to purify treated water obtained from a biological reaction tank for treating sewage or waste water, an air diffusing pipe is installed in a lower portion of the reaction tank to supply dissolved oxygen to the treated water. That is, the dissolved oxygen activates metabolism of aerobic microorganisms, thereby purifying water. In this case, when the size of the reaction tank is increased, a hypoxia region is formed in the lower portion of the reaction tank, thereby lowering purification and treatment efficiency. Therefore, there is still room for improvement in this method and the third method.
The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.